Implantable cardiac pacemakers of varying degrees of sophistication and operational capability are well known in the art. Earlier pacemakers were simple by today's standards, typically being capable of pacing only in a single chamber of the patient's heart, and only at an asynchronous, fixed, and uninhibited pacing rate. Today, pacemakers are available which are capable of synchronous, inhibited pacing in both chambers, at a pacing rate which may be varied according to detected intrinsic cardiac activity or some other physiological indication of the patient's metabolic needs.
Pacemakers are most commonly operated in conjunction with one or more leads, for conveying cardiac stimulating pulses from the pacemaker to the patient's heart, and for conveying electrical cardiac signals from the heart to the pacemaker's sensing circuitry. At least two different types of pacemaker leads, unipolar and bipolar, are commonly known and used.
Unipolar leads have only a single electrode and a single electrical conductor therein. The electrode is disposed at or near the distal end of the lead, which is situated in some particular location in the patient's heart, for example at the apex of the heart in the right ventricle, in the atrial chamber, or in the coronary sinus. The single electrode and conductor of a unipolar lead are used both for sensing (that is, for conducting electrical cardiac signals from the heart to the pacemaker) and for pacing (that is, for delivering stimulating pulses from the pacemaker to the heart.
Bipolar leads have two electrodes and two electrically isolated conductors therein. Often, one electrode, called the "tip" electrode, is a conductive contact disposed at the distal end of the lead, while a second electrode, called the "ring" electrode, is a conductive ring disposed on the lead body some distance back from the distal end of the lead. One of the isolated conductors conducts signals between the pacemaker and the tip electrode, while the other conducts signals between the pacemaker and the ring electrode.
In the case of unipolar pacing and sensing, the electrically conductive pacemaker canister can serve as an indifferent electrode, with pacing and sensing occurring between the lead electrode and the pacemaker canister. In bipolar pacing and sensing, it is not necessary to use the pacemaker canister as an electrode in the pacing or sensing circuit, since pacing and sensing can occur between the tip electrode and the ring electrode, rather than between the tip electrode and the pacemaker canister as in a unipolar configuration.
As pacemaker functionality has become increasingly sophisticated and complex, it has become ever more important for the physician to monitor and obtain information about the pacemaker's operation. Accordingly, many pacemakers today are capable of transmitting, for example via radio-frequency telemetry, information about the pacemaker's current programmable parameter settings and the pacemaker's operational status. In addition, the telemetry system may be capable of transmitting a representation of the intracardiac electrogram. The electrical cardiac signal received on the pacemaker lead and provided to the pacemaker'sensing circuitry can also be applied to the telemetry system, and transmitted in either analog or digital form to an external receiver, where the intracardiac electrogram can be viewed on a strip chart or ECG monitor.
In order to verify or optimize operation of an implanted pacemaker, a physician must be able to determine, among other things, when a stimulating pulse has been delivered to a chamber of the heart, and whether the stimulating pulse possessed sufficient energy to elicit a response from that chamber of the heart (i.e., whether "capture" has been achieved). Such determinations can be difficult to make, particularly with some of the more advanced dual chamber, rate- or activity-responsive pacemakers in which the pacing rate may vary from one cardiac cycle to the next, and in which stimulating pulses may or may not be delivered depending upon sensed intrinsic cardiac activity.
Often, a physician will use a conventional surface electrocardiogram (surface ECG) equipment to monitor cardiac and pacemaker functions. Obtaining a surface ECG can involve placement of a dozen or more skin electrodes, and can be uncomfortable and inconvenient for the patient. In addition, cardiac signals are subject to attenuation and distortion when they pass through the patient's tissue to be received by surface electrodes, and this can complicate the interpretation of the signals and assessment of cardiac and pacemaker function. In some cases, the morphological aspects of an electrical cardiac signal that must be detected to accurately assess cardiac or pacemaker function are simply not revealed in the surface ECG. For example, in determining whether a pacemaker has achieved capture, the physician can look for an often subtle characteristic of the cardiac electrical signal known as an evoked response. Often, evoked responses to a stimulating pulse are not revealed in surface ECG tracings.
Intracardiac electrogram (EGM) signals, derived from the electrical cardiac signal on the pacemaker lead(s) and transmitted to an external programmer as described above, are also known to be useful in monitoring and verifying pacemaker operation. One perceived drawback to such intracardiac EGM signals, however, is that since the same lead is used for pacing and sensing, the high stimulating pulse voltage spike, after-potentials, and electrode-tissue polarizations render the intracardiac EGM system "blind" to the cardiac signal for a period of time immediately following the delivery of each stimulating pulse. Unfortunately, it is during this time period immediately following a stimulating pulse that is of most interest in determining whether capture has been achieved (i.e., whether there has been an evoked response).
Typically, in dual chamber pacing, atrial and ventricular EGM signals are detected using the same lead configuration (i.e., unipolar or bipolar) as used for pacing in the respective chambers. For example, if atrial bipolar pacing between the tip and ring electrodes on the atrial lead, the atrial tip and ring electrodes will also provide the inputs to the atrial sense amplifier.
With conventional surface ECG electrodes, the electrogram signal viewed by the physician on the ECG monitor or strip chart represents a composite of the atrial and ventricular signals of the heart. In the inventors' experience, it has generally been found that physicians are generally more familiar with this type of ECG waveform than the separate atrial and ventricular signals provided from intracardiac electrodes. Since a surface ECG signal represents both the atrial and ventricular signals simultaneously, the physician can easily perceive the timing relationships between activity in the two chambers, and the relative magnitudes of atrial and ventricular signals. With the separate atrial and ventricular signals provided from intracardiac electrodes, on the other hand, the physician must somehow view both signals at once, such as on a dual-trace ECG monitor or a dual-trace strip chart recorder, in order to ascertain information about the interaction or coordination of atrial and ventricular cardiac activity, and about the operation of the pacemaker.
Thus, although intracardiac electrogram signals offer greater resolution (i.e., less distortion and attenuation of cardiac signals) than surface ECG signals, intracardiac pacing leads are not effective for all purposes, since the above-noted problems of after-potentials and electrode-tissue polarizations render the pacing lead "blind" to electrical cardiac activity immediately following delivery of a stimulating pulse from that lead.
One solution for resolving the trade-off between surface ECG and intracardiac EGM signals was proposed by Brownlee et al. in U.S. Pat. No. 4,387,717. In the '717 patent, Brownlee et al. appear to suggest the use of a separate, large surface area electrode, separate from any pacing electrode, to perform sensing. According to the '717 patent, the large surface area sensing electrode, in conjunction with the pacemaker canister as an indifferent electrode, can provide both atrial and ventricular signals to the pacemaker's EGM amplifier. However, it is noted in the '717 patent that the large surface area electrode must be situated far enough away from the pacing electrode so as to avoid the above-noted problems with after-potentials and tissue polarization.
Another solution that has been proposed in the prior art is described in U.S. Pat. No. 4,585,004 to Brownlee. In the '004 patent, Brownlee apparently suggests the use of a separate "data lead", in addition to the pacing lead, for conveying intracardiac signals to the pacemaker's EGM amplifier. According to the '004 patent, the data lead is provided with two ring electrodes which are electrically coupled to a common lead conductor. It is believed by the inventor's that the need for a separate data lead for EGM sensing is undesirable, especially if the two leads used for dual-chamber pacing must also be implanted.